Here is a garment manufacturing trick
The problem is that your feed dogs push the bottom fabric through, then the top is just dragged along. All machines, new or old do this. No problem. Try this trick. Hold the two pieces together about 12 to 18 inches away from the needle end. I usually go from boob to needle, you choose which is good for you. then lift the strip just a little off the machine bed so it does not touch the machine until just before it goes under the needle. Sew up to your fingers and then readjust. Believe me this works, I do it all the time when sewing strips and strings. I also use this when sewing on batting sometimes. You can use a walking foot which pushes the two pieces at the same rate. This is the same thing with out the walking foot. There is probably nothing wrong with your machine at all. I have a high end one and it is the same for me. Try out this "trick" and I think you will see a difference. The reason you don't see this issue on short seams is because the seam ends before the problem appears.